Phase equilibrium of the quaternary system K2SO4–MgSO4–(NH4)2SO4–H2O at 25 °C

• Determination of the phase equilibrium of K2SO4–MgSO4–(NH4)2SO4–H2O system at 25 °C.
• Dry salt phase diagram of this system was investigated and analyzed.
• A new K–Mg–N compound fertilizers was generated.
• Calculation of the phase equilibrium of the above system by Pitzer model.
• The calculated results are consistent with the experimental data well.

The solubilities of the quaternary system K2SO4–MgSO4–(NH4)2SO4–H2O were measured by isothermal method at 25 °C and their dry salt phase diagrams were plotted. The results indicated that, at 25 °C, the solubility phase diagram of this quaternary system consists of seven crystallization regions, those are single-salt crystallization region MgSO4·7H2O, (NH4)2SO4, and K2SO4, complex salt crystallization region MgSO4·(NH4)2SO4·6H2O, MgSO4·K2SO4·6H2O, solid solution crystallization region (K1−n, NH4n)2SO4 (n:0.143–0.893) and (K1−n, NH4n)2SO4·MgSO4·6H2O (n:0.552–0.973), respectively. The extended Pitzer model was derived and applied to the phase equilibrium calculation of system K2SO4–MgSO4–(NH4)2SO4–H2O and its sub-systems at 25 °C. The results showed that the calculated values were consistent with experimental results well. The study on the quaternary system K2SO4–MgSO4–(NH4)2SO4–H2O provides a foundation for the N–Mg–K–S compound fertilizers preparation.